1. Field of the Invention
The invention relates to an apparatus for scanning an information-carrier disc.
2. Background of the Invention
Apparatus of the type to which this invention relates is known, namely a compact disc player, comprising a scanning device which is movably guided parallel to the plane of the disc. The apparatus includes a positioning mechanism for precisely moving the scanning device in a radial direction of the disc. Such a mechanism includes a gear rack which extends parallel to the plane of the disc in the radial direction and can be driven by a motor via a worm whose shaft extends parallel to the plane of the disc, a worm wheel which is in mesh with the worm, and a further drive member which is coaxially connected to the worm wheel and which meshes with the gear rack. Such an apparatus is known and is commercially available.
The prior-art apparatus comprises an optical scanning device for scanning an optically readable information carrier on which the information is stored in a spiral track and which is generally referred to as a Compact Disc. In such an apparatus the scanning device is movable radially or substantially radially of the information carrier to enable the scanning device to be moved selectively to any desired track portion of the spiral track. For this purpose it is desirable to position the scanning device with the greatest possible precision, which means that the travel of the scanning device per revolution of the motor should be minimal. This is achieved when the reduction ratio of the gear mechanism for driving the scanning device is as large as possible. Moreover, the gear mechanism should be as compact as possible in order to minimise the dimensions of the apparatus. In the prior-art apparatus the further drive member of the gear mechanism is a spur wheel, enabling only a comparatively small reduction ratio to be attained, which is in conflict with the requirement to position the scanning device with the greatest possible precision. To achieve a large overall reduction ratio of the gear mechanism the diameter of the worm wheel should therefore be comparatively large in order to accommodate a sufficiently large number of teeth for a large reduction ratio, but this is unfavourable for a compact construction of the gear mechanism.